The invention relates to a preparation for a magnesium ammonium phosphate cement, a process for the production of such a cement, and uses of the so prepared cement.
The healing process of bone defects, the extent of which exceeds a critical size (critical size defects, CSD), requires the use of application site specific transplants or implants to prevent growing of connective tissue into the defect and to restore the lost biofunctionality. Regarding the growing-in behavior and the functional restoration of the autologous transplantation of a body's own bone represents a “gold standard”; however, with large defects it has the disadvantages of lacking availability and the risk of a repeat operation. Therefore, in recent years there were investigated synthetic bone replacement materials. In the clinical application these synthetic bone replacement materials have to fulfill different criteria. In a load-bearing area mechanical properties are mainly the focus and primarily high-tensile metallic materials such as titanium, titanium alloys, stainless steels, and CoCr alloys are used. Importance is attached to a connective tissue free growing in of the material, and a selective resorption and replacement by natural bone in the not or only slightly load-bearing area. Artificial materials with clinical application for such functionally non-load-bearing defects are calcium phosphate ceramics and cements, respectively, bio-glasses, collagens, or lyophilized bone implants. At the application site the materials have to absorb mechanical load only partly and as the mostly porous lead structure should support growing in of new bone.
A main clinical requirement to bone replacement materials is their resorptivity in the physiologic environment regenerating native bone substance. Many of the mentioned materials cannot or only partially fulfill this requirement. For example, sintered ceramics of hydroxyl apatite, or cements of polymethyl methacrylate do not show physiological degradability. Degradable materials with clinical employment are for example tricalcium phosphate ceramics as powders or granules which however, cannot take over a support function in the bone defect. Additionally, in situ formable and hardening mineral cements are known that on the one hand can set to hydroxyl apatite and that only are resorbed over longer periods of time or compositions that after setting consist of calcium hydrogen phosphate·dihydrate (brushite) and can be resorbed medium-term due to their comparatively high solubility in a period of about 3 to 6 months. Reservations regarding a wide-ranging clinical application of such brushite cements result from the comparatively low mechanical strength, as well as the strongly acidic pH value of the materials during and after the setting process that can result in the release of acidic phosphate ions into the surrounding tissue.